With reference to FIG. 1 and as commonly known in the industry, steam assisted gravity drainage (SAGD) uses a well-pair of closely coupled, horizontally-extending, generally parallel wells comprising a first steam injection well (injection well) and a second production well (production well) spaced and positioned below the injection well. Typically, SAGD is commenced in a start-up phase by independently and simultaneously circulating steam through both the injection well and the production well. Steam is injected through a tubing string which extends to a toe of each of the injection well and the production well. The injected steam condenses in each well, releasing heat and creating a liquid phase which is removed through the casing-tubing annulus in the opposite direction of the injected steam.
The released heat is conducted initially through an intervening portion of the formation between the injection well and the production well (inter-well region) and then through the formation to sufficiently heat and otherwise mobilize bitumen therein to cause the heated bitumen to flow by gravity drainage into the production well. In this start-up phase, a thermal chamber is created between the injection well and production well as the mobilized bitumen gravity drains into the production well.
After a well-to-well steam communication of is achieved, steam is injected continuously into the upper injection well and condensate and heated oil are removed from the lower production well.
This start-up of SAGD has been enhanced to date through various known techniques including cold water dilation, steam dilation, solvent soaking and electrical heating for reducing the time required for establishing communication between the injection well and the production well. In cold water and steam dilation, cold water or steam is injected into the inter-well region for creating a vertical dilation zone and increasing porosity, permeability and water saturation of the inter-well region.
In solvent soaking, a solvent is injected into the inter-well zone and allowed to soak prior to steaming. The solvent mixes with the bitumen therein and reduces the viscosity of the bitumen allowing the bitumen to be mobilized at a lower temperature.
In electrical heating techniques, an electrical downhole heater is placed in the wells for conducting heat into the inter-well region to reduce the viscosity of the bitumen therein.
As the mobilized bitumen drains into the production well, interstitial space voided by the mobilized bitumen forms a steam chamber which continues to grow horizontally and vertically. Simultaneous circulation of steam into both the injection well and the produce well (or SAGD start-up) is ceased when the steam chamber reaches the production well, and ramp-up of SAGD can begin.
During ramp-up, steam in injected into the injection well only, at a constant pressure for mobilizing heavy oil above the injection well for continued gravity drainage and recovery at the production well.
Factors dictating the success or timeliness of enhanced oil recovery of hydrocarbon-bearing formations include the transport of thermal or drive mechanisms into the formation for enhanced oil recovery (EOR). Often, primary extraction of hydrocarbons leaves areas of voidage, wormholes or other areas of high transmissibility conducive to introducing EOR mechanisms.
In formations generally deemed suitable for SAGD, such as previously un-exploited formations, the initial transport conditions for steam, solvent or other transmission means are slow to initiate and can retard the development of a thermal mobilization chamber. Further, to date, each well-pair of a field of well-pairs is treated independently without consideration or advantage of adjacent well-pairs.
Regardless of the mechanism, there is an opportunity to improve initiating circulation for steam assisted gravity drainage and inter-well communication between injection and production wells.